1. Field of the Invention
This invention relates to insulating structures and, in particular, to insulating structures for use in electrical systems in atmospheric, gas-insulated or liquid dielectric environments, such as insulators, bushings, spacers and dielectric housings for high voltage devices.
2. State of the Art
In general, the integrity of insulating structures that are exposed to surface pollution or moisture may be prejudiced by electrical discharges across non-conducting bands that can lead to damage and/or flashover.
Insulating structures for outdoor and industrial applications generally consist of axi-symmetric shapes that usually include umbrella-type sheds in their design. These sheds are designed to increase the longitudinal surface (creepage) length in order to achieve a given withstand voltage level and to mitigate the effects of precipitation.
The substantial size of insulators, bushings and dielectric housings for high-voltage devices which are used in ambient environments, whether indoor or outdoor but especially in industrial or coastal sites, arises mainly from the large values of surface creepage length (mm/kilovolt) which are needed for safe insulation performance when they are polluted. Although a dry layer of pollution (whether industrial pollutants or saline deposits) normally has little effect upon the dielectric strength of the insulating structure, problems arise when the pollution layer becomes wet under fog or light rain. The conductivity of the wetted surface of the structure leads to a leakage current which, although itself generally not harmful, can often cause partial drying which encircles the surface (dry bands). A large proportion of the voltage applied to the insulator will appear across the band with consequential damage from electrical breakdown (partial arcs or complete flashover).
The importance of good pollution performance of insulating structures is of such significance that international standards specify high-voltage laboratory test procedures (salt-fog and clean-fog tests) to achieve agreed specifications.
In the past, low, medium and high voltage insulators have been made generally of porcelain or glass. Such materials are highly insulating in relatively dry environments. However, the surface resistance of such materials tends to decrease by around four or five orders of magnitude in polluted, wet or humid conditions, thereby substantially reducing their insulating properties. Further, the heavy, brittle nature of such materials makes them vulnerable to accidental damage and vandalism, and, in addition, collection of pollutants on the porcelain or glass outer surface can result in flashover or arcing as well as unacceptably high leakage current from one end of an insulator terminal to the other.
In order to limit discharges across dry bands, particularly for use in severe environments, some types of insulating structure have a semiconducting glaze applied thereto. However, although this solution provides some improvement, it does not successfully eliminate partial arcs.
Polymeric materials, such as ethylene propylene diene monomer (EPDM) and silicone rubber are finding increased application in the manufacture of insulators and other high voltage equipment. Compared with long-established porcelain and glass structures, they have (with glass-fibre reinforcement) a superior strength-to-weight ratio, are less environmentally obtrusive, and are less vulnerable to accidental damage or vandalism.
More importantly, such materials can contribute to improved equipment design due to their good dielectric performance, particularly under polluted conditions. This is due to the natural hydrophobicity of polymeric materials which prevents the occurrence of a continuous wet surface, thereby inhibiting leakage currents and the formation of dry-band arcing. It is well established that the hydrophobic property of a clean polymeric surface is transmitted to an overlying layer of pollution, probably as a result of diffusion of oily constituents through the layer.
U.S. Pat. No. 5,830,405 describes a tubular polymeric shed comprising a central tubular portion surrounding an elongated core. A plurality of radial wall ring fin extensions extend from the central tubular portion and a skirt line extension (or “shed”) to increase creepage length and reduce partial arcs. However, this solution does not successfully eliminate partial arcs.
FIGS. 1 and 2 of the drawings represent a portion of a conventional insulating structure 100 showing a single shed 102 and part of the insulating shank 104. When the structure is carrying a longitudinal surface current I under adverse conditions, the current density J (in amperes/m2) is non-uniform even where the pollution layer is of uniform conductivity σ (Siemens/m) and thickness T. This is because the radius r and therefore the circumference S of the circular surface contours varies along the sheds of the structure.
In this case, the current density in the pollution layer is given by:J=I/(ST)=I/(2πrT)
For this uniform pollution condition, the surface electric field E (in volts/m) is also longitudinal and non-uniform, and is given by:E=I/(σST)=J/σ
Heating of the moist pollution layer is non-uniform, thereby causing dry bands to form. The power density dissipation P (watts/m3) of such surface layer heating is given by:P=EJ=J2/σ=I2/(σS2T2)
This equation indicates that the greatest heating of a uniform pollution layer will occur on the insulating structure in the region of the smallest contour perimeter S(min). Dry bands will thus most easily form at the shank 104 of the structure. As a result, in the case of conventional polymeric insulators, bushings and housings which employ such non-uniform profiles, it has been found that such polymeric structures frequently fail because of damage in the shank region where partial-arc activity is greatest.
Further, research is continuing concerning the long-term durability of polymeric materials. Ageing and degradation occur which can adversely affect the surface condition of the materials and cause loss of hydrophobicity, and the occurrence of dry-band partial arcing could more easily result in tracking or surface erosion than is the case for the traditional inorganic structures, which is clearly unacceptable.